This invention relates, in general, to digital electronic memories and, more specifically, to digital radio frequency memories (DRFMs) suitable for use in radar countermeasures equipment.
Active radar jammers are used in the field of electronic countermeasures to confuse or counter a system originating radar signals. In some situations, it is desirable to return signals to the radar system which are exact copies of the arriving radar signal. In other situations, it is desirable to return signals to the radar system which have characteristics other than that of the received radar signal in order to further confuse the radar system. In any event, it is usually necessary for the countermeasure system to store the received radar signal and reproduce it at a later time.
Previously, delay lines of various types have been used to effectively store the received radar signal for a short period of time and make the stored radar signal available at a later time. Typical delay lines, however, have the disadvantage that the delay cannot be electronically changed easily, and it is difficult to obtain reasonably long delay periods without serious signal degradation. An improvement over the delay line technology has been achieved by the use of digital radio frequency memories (DRFMs) which convert relatively high radio frequency (RF) signals down to a lower intermediate frequency (IF) by mixing the RF with a local oscillator (LO) signal for storage into a digital memory device. The digital memory can be controlled in a manner similar to the control of the digital memory of a computer. Stored values representing the radar signal can be recalled and reproduced at any time delay desired. Further, manipulation of the digital values to produce changes in the replicated signal are also conveniently done by digital processes.
U.S. Pat. No. 4,713,662 entitled "Modulated Digital Radio Frequency Memory", which issued Dec. 15, 1987 in the name of Richard J. Wiegand, the inventor herein, and assigned to Westinghouse Electric Corporation, the assignee herein, describes single and dual channel DRFMs. In either system it is necessary to first convert the RF signal down to a lower IF signal which is manageable. The IF signal is digitized by means of an analog-to-digital (A/D) converter at a given sampling rate which is determined by the capacity of the A/D equipment. The digitized signal is then stored in a digital memory. At a later time the digital signal may be called from memory and converted to an analog IF signal by a digital-to-analog (D/A) converter. The IF signal is mixed with the local oscillator (LO) signal to reproduce or convert the IF to the higher RF frequency signal which is a replication of the incoming radar signal.
Although conversion of the incoming radar signal to a lower frequency IF signal allows more realistic A/D and D/A equipment, a consequence of mixing signals of different frequencies is the production of two resulting signals which represent the sum and difference of the original signal. In many situations the sum and the difference signals are easily distinguished. For example, if a 3100 MHz RF signal is mixed with a 3000 MHz LO signal the sum and difference signals produced are 6100 MHz and 100 MHz. These Signals are easily distinguished. Within the DRFM the 6100 MHz signal is ignored and the relatively low 100 MHz signal is digitized and stored in the memory. However, when the 100 MHz signal is recalled from memory and converted up to the RF level by mixing with the 3000 MHz LO signal, the original 3100 MHz RF signal representing the sum is produced as well as a 2900 MHz spurious image signal which represents the difference. The image signal has nominally the same amplitude and is not easily distinguished from the original signal. Thus, it is necessary to suppress the image signal.
The usual method of suppressing the image signal in DRFMs is to employ two channel (quadrature) I&Q memory system described in the above noted Wiegand patent. By using the two channel I&Q system with proper phase shifting techniques the image signal can be eliminated or cancelled at the output. However, I&Q systems have additional components which are expensive and heavy. Also, the two channel system contains a non-performance region, or hole, which occurs when the frequency of the RF signal is close to the frequency of the local oscillator (LO) of the memory system.
According to the well known Nyquist sampling theory, the maximum usable instantaneous bandwidth (IBW) of a memory system is equal to one-half the sampling rate of the A/D and D/A converters used in the DRFM. Having a large instantaneous bandwidth (IBW) is advantageous from the standpoint that it allows radar signals over a wider range to be detected, stored and jammed by the countermeasures equipment. One way to maximize instantaneous bandwidth is to use one bit sampling. An example of one bit sampling is a system which looks at or samples a high frequency signal by registering (storing) the polarity of the signal at each sample point.
One bit sampling also provides the advantages of increased amplitude dynamic range and reduced storage requirements. However, one bit sampling results in a large number of spurious frequencies or unwanted spectral lines (spurs) being produced in the IF signal. The spurs are ultimately reproduced in the RF output signal because of the mixing processes used in the DRFM. Spurs degrade system performance and should be suppressed by some additional means.
According to the prior art prior to Wiegand's invention, the most effective way to obtain reasonably large bandwidths and suppress images was to use the two channel I&Q storage system.
Wiegand's patent describes a single channel DRFM. According to , modulation of the frequency or phase of the LO eliminates one channel. Also, modulation suppresses spurs, and eliminates the hole or nonperformance region. One bit sampling is maintained.